The Influence of Selection and Temperature On a Mutant Character (CiD) in Drosophila Melanogaster

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1. In the mutant ciD of Drosophila melanogaster the relative length of the 4th vein was used as a quantitative character. Divergent selection during 30 to 40 generations for long and short vein length was practised in three unrelated base populations. 2. Selection progress continued through about 20 to 30 generations, sometimes with alternating periods of advance and stability. In all three base populations heritability was approximately 0.3, and similar selection limits were reached far beyond the ranges of the base populations. 3. During the course of selection there were striking changes in the appearance of the frequency distributions (bimodality), particularly in the lines selected for a long, i.e. more normal vein. 4. An explanation for this is proposed, which is based on the assumption that the factors (genetic or environmental) influencing the expression of a character are in their effect dependent on the relative length of the vein. Temperature experiments are described which strongly support this hypothesis. Expression, in response to temperature, appeared to be very sensitive to change in a particular expression range. 5. This view implies that vein determination is governed by two at least partially independent genetic systems, one controlling the relative length of the vein and the other relating modifying influences to their phenotypic effects. 6. Thus, the variability in this mutant character seems to be governed in large measure by the properties of its developmental system. 7. The results are discussed in terms of their significance in the fields of quantitative genetics, developmental genetics, and evolution.